1. Field of the Invention
The present invention relates to a quartz crystal unit with high stability, which uses a quartz crystal blank such as an SC-cut crystal blank, an IT-cut crystal blank, an AT-cut crystal blank, or the like. In particular, the present invention relates to a holding structure for a crystal blank in the crystal unit in which supporters are used.
2. Description of the Related Arts
A crystal unit having a structure in which a quartz crystal blank is hermetically encapsulated inside a container is highly stable in terms of its vibration frequency. Such crystal units are being used as frequency sources for radio devices at base stations in communication networks, for instance. A crystal blank that configures such a crystal unit can be categorized into several types of “cut” based on the crystallographic orientation in cutting out a crystal blank from a single crystal of quartz. Typical types that are known, for example, are X-cut, AT-cut, BT-cut, and so forth. With respect to a crystal of quartz, three crystal axes that are X-axis, Y-axis and Z-axis are crystallographically defined.
In order to retain the frequency stability of the crystal unit, with high precision particularly, without the crystal unit being influenced by the ambient temperature, a kind of a mechanism which could heat the crystal blank by a heater and keep the temperature of the crystal at a certain temperature is used. The crystal unit, of which temperature is kept at a certain temperature by a heater or the like, is used in an oscillator of a communication device, such as a device for a radio base station, for instance. As a crystal unit to be used for that purpose, there is a kind that holds a crystal blank of a type such as an SC-cut, IT-cut or AT-cut in a horizontal direction with respect to a metal base, the crystal blank being hermetically encapsulated inside a container composed of the metal base and a metal cover.
FIG. 1A is a plane view showing one configuration example of a conventional crystal unit in which a crystal blank in the crystal unit is kept in a horizontal direction with respect to a metal base. FIG. 1A shows the manner of holding the crystal blank in a state without a metal cover. FIG. 1B is a partial sectional view showing a part of the crystal unit that holds the crystal blank. FIG. 1C is a perspective view of a supporter used in the crystal unit. Details of the structure of the crystal unit shown in these figures are described in Japanese Patent Laid-Open No. 11-308067 (JP-A-11-308067), for instance.
In this crystal unit, crystal blank 1, of a disk-like shape for instance, is supported over metal base 2 through lead wires 6 and supporters 7. Lead wires and supporters 7 will be described later. Crystal blank 1 is hermetically housed inside a space between metal base 2 and a metal cover (not shown) while the metal cover is applied over metal base 2. Metal base 2 also has a disk-like shape.
Crystal blank 1 is an AT-cut quartz crystal blank, for instance, and includes excitation electrodes 3a, 3b of approximately circular shapes at central portions of both principal surfaces thereof, respectively. Extraction electrodes 4a, 4b extend from excitation electrodes 3a, 3b, respectively, toward an outer circumference of crystal blank 1. Here, an extending direction of extraction electrode 4a and an extending direction of extraction electrode 4b are apart by 120 degrees when seen from a center of crystal blank 1. Extraction electrodes 4a, 4b having been extracted to two parts of the outer circumference of crystal blank 1 further extend to an end face of crystal blank 1. The AT-cut crystal blank, considering X-, Y′- and Z′-axes of the crystal of quartz which are derived by rotating the three axes (i.e., X-, Y- and Z-axes) by 35 degrees with respect to the X-axis, will be a crystal blank having a principal surface which is a plane formed by X-axis and Z′-axis. The AT-cut crystal blank vibrates in a thickness-shear vibration mode.
At metal base 2, three lead wires 6 are arranged equally at 120-degree intervals when seen from a center of metal base 2, and these lead wires 6 are arranged in a way penetrating through metal base 2. Metal base 2 are provided with through-holes at the positions where respective lead wires 6 are penetrating through. Lead wire 6 is inserted to each of the through-holes. As shown in FIG. 1B, a space between lead wire 6 and metal base 2 in each through-hole is filled with glass 5, whereby each through-hole is hermetically sealed and lead wire 6 is electrically insulated from metal base 2. In this way, lead wire 6 and glass 5 form an airtight terminal at each through-hole.
To an apical end of each lead wire 6, supporter 7 as shown in FIG. 1C is connected. Supporter 7 is made with metal and has a tabular shape. A plate surface of supporter 7 extends perpendicularly with respect to metal base 2. Each supporter 7 is provided with slit 8 and tongue portion 9. Slit 8 is formed as being opened upwardly in an upper end thereof. Tongue portion 9 is formed at a lower end of slit 8 in a way bending at a right angle and extending perpendicularly with respect to the plate surface of supporter 7.
Crystal blank 1 is mounted on tongue portions 9 of supporters 7, at two places in the outer circumference of crystal blank 1 where extraction electrodes 4a, 4b are extended to, and at another different place in the outer circumference of crystal blank 1 which is apart from each of the other two places by 120 degrees, which makes a total of three places in the outer circumference of crystal blank 1. Thereby, over metal base 2, crystal blank 1 is held in a way such that the main surface of crystal blank 1 is parallel to metal base 2. Here, the end face of crystal blank 1 is to contact closely to slits 8 at respective positions of supporters 7. A concave portion formed by a lower end surface and both side surfaces of each slit 8 is to be filled with conductive adhesive 10, whereby crystal blank 1 is fixed to each of supporters 7 through conductive adhesive 10.
Such a crystal unit with high stability is to be mounted on a wiring board along with circuit elements composing an oscillation circuit and with other elements, and heated by a heater structure (not shown) to be maintained at a certain temperature. By maintaining the crystal unit at a certain temperature, the vibration frequency of the crystal unit will exhibit extremely high stability. More specifically, the amount of change in the vibration frequency will be kept to 1 ppb (i.e., one part per billion) or less, for instance. The wiring board with the crystal unit mounted thereon in the above-described manner can be placed inside a cassette, for instance, to be set in communication equipment used in a radio base station.
In the above-described crystal unit, crystal blank 1 with a disk-like shape is held at three points by supporters 7 being arranged equally at 120-degree intervals. In geometry, a plane is to be specified by three points that are not in the same straight line. With this in consideration, the above-described structure of the crystal unit can hold the crystal blank with little shakiness and most stably in geometric perspective. Accordingly, with respect to the holding system of the crystal blank, little deformation is caused even when possible impact strikes from outside. With this crystal unit, it is possible to maintain a good vibration characteristic with the crystal blank. Moreover, due to conductive adhesive 10 that fills inside of the concave portion of each slit 8, it is only the end face of crystal blank 1 that is to be connected with supporters 7 at the positions where supporters 7 are arranged. It is possible to keep the contact areas of supporters 7 with respect to crystal blank 1 small, whereby dissipation of vibration energy from crystal blank 1 toward supporters 7 can be kept small. Thus, desired vibration intensity can be obtained at crystal blank 1.
In the above-described configuration of the crystal unit, an equally-spaced three-point holding structure, which is geometrically most stable, is used However, crystal blank 1 is fixed to each supporter 7 through a little amount of conductive adhesive 10 that fills inside of the concave portion of slit 8 which is formed in each supporter 7 as being opened in the upper end. With this crystal unit, there is a problem in that the crystal unit will be greatly influenced by an impact that could strike at a time when a cassette containing the crystal unit is to be set in a communication device, or the like. More specifically, when impact from outside strikes the crystal unit, the holding state of crystal blank 1 by conductive adhesive 10 will change, which could result in changing the vibration frequency of crystal blank 1. This indicates that a stability of frequency with respect to the crystal unit is not good against impact.
In this case, conductive adhesive 10 at each supporter 7 is connected only with the outer circumference end face of crystal blank 1 at the concave portion inside slit 8. Therefore, when a comparatively large impact, such as an impact by a large earthquake, for example, strikes the crystal unit, there is a possibility that conductive adhesive 10 exfoliates from crystal blank 1. The crystal unit with possible exfoliation of the conductive adhesive may not be positively sufficient in terms of its use in a radio base station that requires high stability. With respect to a crystal unit with high stability, frequency change over 1 ppb (ire., one part per billion) with respect to the original vibration frequency is considered as a problem. Therefore, for the crystal unit with high stability, not only the vibration characteristic under static condition, but also an impact resistance characteristic taking in the perspective of possible exfoliation, etc. is considered significant.
As a background art of the present invention, Japanese Patent Laid-Open No. 2002-261566 (JP-A-2002-261566) discloses a crystal unit in which a crystal blank is held by fixing an end face of the crystal blank to supporters by metal brazing, while the supporters are provided with slits but not with tongue portions. Japanese Utility Model Laid-Open No. 53-57777 (JP-U-53-057777) discloses a crystal unit that uses supporters with slits, an upper end portion of each slit being closed.